The present invention is in the field of emissions control and relates more particularly to improved catalytic converters for the abatement of carbon monoxide, nitrogen oxides, and unburned hydrocarbons generated by internal combustion engines such as used in stationary and mobile applications, e.g., in automobiles.
At present, the majority of catalytic converters for automotive engine emissions abatement applications comprise one or more platinum group metal catalysts dispersed on a refractory high-surface-area coating, the coating and catalyst being supported within the channels or "cells" of a ceramic honeycomb support through which the engine exhaust stream is passed. Ceramic honeycomb supports useful for this application, conventionally comprising an inlet face, an outlet face, and a plurality of parallel open-ended channels or cells traversing the support between the inlet and outlet faces, the channels being defined by interconnecting, criss-crossing cell walls, are described in U.S. Pat. No. 3,885,977. Methods and apparatus for the production of such supports by ceramic powder extrusion processes are disclosed in U.S. Pat. Nos. 3,790,654 and 4,731,010.
Past theoretical work in the area of extruded cellular ceramic supports has concentrated on pressure drop and conversion efficiency. Conventionally, conversion efficiencies have been considered to be directly related to the geometrically calculated surface areas of the cellular supports, i.e., to the total of all of the surface areas of all of the walls forming the channels of the cellular (honeycomb) supports. Thus designers in this field have customarily based predictions of the catalytic performance of each commercial square-celled extruded product largely on the calculated geometric surface area of the support. As data have accumulated, however, discrepancies between the calculated surface areas and measured catalytic performances have raised doubts about the exact relationship between these two parameters.
A particular example of such a discrepancy is illustrated by performance comparisons between a 400-cell extruded ceramic honeycomb (i.e., a honeycomb having a cell density of 400 cells per square inch of honeycomb inlet surface area) and a 400-cell wrapped metal honeycomb. The geometric surface area of the metal honeycomb is approximately 33% higher than that of the ceramic honeycomb, due largely to the sinusoidal shape of the cells or channels in the metal honeycomb. However, in tests of the emissions performance of the two products, conducted at the same substrate volume (i.e., with the metal honeycomb maintaining a surface area advantage of 33%), the performance of the two products is found to be virtually identical.
Past approaches to improve the performance of catalytic converters by increasing honeycomb geometric surface area have a number of disadvantages. First, significant surface area increases require increases in cell density. Such increases tend to reduce the cell hydraulic diameters and increase gas pressure drops across the converters, even though some reductions in cell wall thickness to reduce converter pressure drop can be made. Reductions in cell wall thickness, however, are limited by product strength requirements and other considerations, including increased difficulty of manufacture which can greatly increase the cost of the products.
These and other concerns suggest that a deeper understanding of the material and/or geometric factors affecting emissions performance will be required if significant improvements in the performance of honeycomb catalytic converters for gas treatment applications are to be secured.
It is therefore a principal object of the present invention to provide catalytic converters of improved performance, by specifying the design parameters of the honeycomb substrate in light of a further analysis of the factors governing converter performance.
Other objects and advantages of the invention will become apparent from the following description thereof.